Sea of Thieves is Rare’s riskiest voyage yet, but it’s a huge success

Chinese soft-shelled turtles mainly urinate from their mouths

A team of biologists has published a paper revealing that when Chinese soft-shelled turtles mysteriously dunk their heads in puddles for up to 100 minutes a time, they are in fact urinating from their mouths. More specifically, they're excreting most of their urea this way.

The turtle, known as Pelodiscus sinensis, is a rather unusual specimen all round. Though it generally breathes through its lungs, it has a series of buccopharyngeal villiform processes (BVP) at the back of its mouth, which behave a little like gills and look like rows of tiny studs. It was suspected that when it dunked its head in water from time to time, it could be engaging in respiration similar to that of frogs (buccopharyngeal respiration), enabling it to thrive in the swamps and marshland it is found in across Asia. In reality, the turtle spends most of its time above water and has a pointy snout and large tube-like nostrils so that it can snorkel its way around the marshes. So, if it has evolved in such a way, why does it insist in dunking its head in puddles when its habitat has dried up?

This is the question Yuen K Ip of the University of Singapore set about answering, working on the premise that the turtle's gill-like features could be used, like in some fish, to excrete nitrogen via urea.

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Ip and his team bought (/saved) a few 200-400g specimens from the China Town wet market in Singapore -- turtle soup is popular in the region -- and were kept them in freshwater-filled plastic tanks. By attaching dialysis clips and latex tubing to the turtles' cloacae, the team could collect the urine for analysis. They immediately found that very little urea was being excreted via the cloaca, and was thus not being processed by the kidneys very often.

To find out what was being excreted from the mouth, the team perched the turtles beside a tank of water which acted as the "puddle" for them to dunk their heads in. They would spend between 20 and 100 minutes with their heads submerged and, while this was occurring, the team witnessed the turtles engaging in a "rhythmic throat movement characteristic of buccopharyngeal respiration"; the turtles were breathing, as well as swirling the water around their mouths. After submersion, the team measured the urea present in the tank and found that it was up to 50 percent higher than the rate of excretion via the cloaca. The turtles were simultaneously breathing, and contracting their throats to excrete urea from their mouths.

To verify the find, the team then injected urea into the turtles to see how it was processed. Comparisons of blood and saliva samples revealed that a massive 250 times more urea was present in the saliva.

The practice might sound bizarre but Ip believes the turtles developed the technique in order to survive in marshes where the water's salt levels would be too much to digest in the normal fashion -- in the same way that humans do not have the faculties to digest seawater, too much would leave the turtles dehydrated and constantly having to drink, while their blood would be flooded with urea, becoming toxic. "Because the buccopharyngeal urea excretion route involves only rinsing the mouth with the ambient water, problems associated with drinking brackish water and the consequential disruption of ionic homeostasis can be avoided," explains the paper.

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Believing that if the same process could be induced in humans kidney failure patients would no longer have to undergo dialysis, Ip began searching for the responsible gene. Urea transporters -- proteins mammals carry to transfer urea across membranes -- can be suppressed by phloretin. When this was added to the puddle of water, the team found the turtles could no longer excrete urea via their mouths. Further investigation revealed evidence of the urea transporter in the mouth and BVPs, but not in the kidney, as is the norm. Ip imagines that, if a urea transporter could somehow be introduced in the same way to a kidney failure patient, one day those patients would not have to have their blood cleansed -- the toxins would come straight out their mouths.